1. Field of the Invention
The present invention relates generally to the field of acoustic gas monitoring and more specifically to monitoring, at various pressures, of gas mixtures used for diving, medical purposes, or in related industries.
2. Description of the Prior Art
In many applications it is necessary to determine the composition of gas mixtures used for breathing or for anesthesia purposes, for example. Conventional techniques typically employ wet chemistry, mass spectrometry, infrared techniques, galvanic fuel cells, or even paramagnetism.
In many situations, such as with breathing mixtures, one is concerned only with two component gases. When this is the case, one doesn""t need a measurement system specific to a particular gas, since the gases are specified, and it is only the fraction that needs determination. In other cases, it is possible to treat as binary gases, mixtures which aren""t binary. This occurs when some of constituent gases are present in fixed rations. An example of this would be a mixture of dry air plus pure oxygen. The composition of dry air is known, so the addition of pure oxygen allows the resulting mixture to be analyzed by binary gas technique.
A very important area of gas monitoring is in general for breathing gases. In medical applications, oxygen is frequently added to air. Sophisticated SCUBA diving is now done with NITROX, which is oxygen enriched air. The existing inexpensive oxygen monitors in use are typically electrochemical devices, either fuel cells or polarigraphic half cells. Those devices have a limited life time and require frequent recalibration. More importantly, such prior art devices have severe temperature dependence, and can only be approximately compensated for temperature. In diving or other hyperbaric applications, the fuel cells are commonly used. These devices have semi-permeable membranes, which frequently malfunction under pressure, and result in false indications. Further, the membranes, if they become wet, will now sense the dissolved oxygen level in the surface liquid rather than in the gas mixture. For these reasons, it is common practice to use triple redundant fuel cells in rebreathers for divers.
Acoustical methods have been employed for at least a century for gas analysis. An early application of acoustic technique to gas analysis was the detector used in mines at the beginning of the 20th century. This device was an automatically actuated double whistle, one unit filled with pure air, the other with the air from the mine. The velocity of sound in a gas is function of temperature and gas composition. Since the whistles were of identical construction and at equal temperature, any difference in pitch of the two whistles was due to potentially dangerous contamination of the mine air supply. Very small differences in frequency could be perceived as a xe2x80x9cbeat notexe2x80x9d. Thus the extent of explosive and/or toxic fumes could be estimated.
Most existing acoustic methods of gas analysis have used either time-of-flight method (see, e.g., U.S. Pat. Nos. 5,060,506, 5,351,522, and 5,625,140) or phase shift method (see e.g., U.S. Pat. No. 3,805,590). In fact, the time-of-flight patents are quite concerned with minimizing sound reflections and standing waves. The time-of-flight methods are inherently pulsed, since a time interval must have a beginning and an end. U.S. Pat. No. 3,805,590 is concerned with a binary mixture of helium and oxygen where the speed of sound varies greatly as a function of concentration, allowing very approximate measurement techniques to provide adequate performance. The pulsed time-of-flight methods may require multiple calibration points to compensate for the variation in the response of the transducers.
These existing devices are also sensitive to the ambient pressure, as the acoustic impedance of the gas changes linearly with density. Pulse devices also suffer from an inherently low signal to noise ratio, as the acoustic energy in a short pulse is small. This low signal-to-noise ratio may cause measurement errors. Further, the resonant piezoelectric transducers in many of these devices are themselves not temperature stable, and may even suffer drift over a time period, requiring-re-calibration.
It is therefore desirable to introduce and develop a new method for analyzing gas mixtures.
The present invention is a unique and novel method for analyzing gas mixtures.
Described generally, the present invention is an apparatus for gas mixture analysis. The analyzing apparatus includes a symmetric acoustic resonating device for generating quarter-wave mode acoustic resonance in its hollow chamber which is filled with a gas mixture.
The symmetric acoustic resonating device includes a pair of parallel elongated tubes each having a hollow chamber with two opposite open ends. The symmetric acoustic resonating device also includes a pair of receiving transducers each mounted at an aperture located at the middle of a respective one of the two elongated tubes, such that the pair of receiving transducers are adjacent to each other, and a pair of transmitting transducers each mounted at an aperture also located at the middle of a respective one of the two elongated tubes, such that the pair of transmitting transducers are opposite to each other. The symmetric acoustic resonating device further includes an electronic circuitry connected between the pair of receiving transducers and the pair of transmitting transducers for producing a desired acoustic resonance in the hollow chambers of the elongated tubes for determining the composition of gas mixtures contained in the hollow chambers.
The analyzing apparatus also includes a temperature sensor for measuring the temperature of said gas mixture. The analyzing apparatus further includes a counter-timer for measuring the frequency of said acoustic resonance. The temperature and frequency measurements are used for analyzing the composition of the gas mixture.
Further novel features and other objects of the present invention will become apparent from the following detailed description, discussion and the appended claims, taken in conjunction with the drawings.